1. Field of the Invention
This invention is an improvement in multi-layer coatings and methods of coating which utilize catalysts in one layer and blocked isocyanates in an adjacent layer.
2. Description of the Related Art
Modern automotive and appliance finishes are frequently two-layer coatings where a pigmented base coat is enhanced by covering with a clear top coat. In such multi-layer systems it is highly desirable that the top coat adhere strongly to the base coat and that the top coat not develop objectionable color.
Top coat color development and/or poor adhesion of the top coat to the base coat is a particular problem in two-layer coatings wherein one layer contains an acidic component and the adjacent layer contains a blocked isocyanate. This situation typically occurs where the base coat is crosslinked with amino resin type crosslinkers using acid catalysts and the top coat is a "one-package" polyurethane coating containing a blocked isocyanate wherein the blocking agent is released during heat curing. Surprisingly, it has been found that the addition of an amino resin with at least one imino-functional (--NH--) group to curable compositions which form polyurethane top coat layers adjacent acid-cured amino resin coating layers is highly beneficial for improvement of color and/or adhesion.